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// Copyright 2018 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package fdpipe
import (
"io"
"os"
"syscall"
"time"
"gvisor.googlesource.com/gvisor/pkg/fd"
"gvisor.googlesource.com/gvisor/pkg/sentry/context"
"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
"gvisor.googlesource.com/gvisor/pkg/syserror"
)
// NonBlockingOpener is a generic host file opener used to retry opening host
// pipes if necessary.
type NonBlockingOpener interface {
// NonBlockingOpen tries to open a host pipe in a non-blocking way,
// and otherwise returns an error. Implementations should be idempotent.
NonBlockingOpen(context.Context, fs.PermMask) (*fd.FD, error)
}
// Open blocks until a host pipe can be opened or the action was cancelled.
// On success, returns fs.FileOperations wrapping the opened host pipe.
func Open(ctx context.Context, opener NonBlockingOpener, flags fs.FileFlags) (fs.FileOperations, error) {
p := &pipeOpenState{}
canceled := false
for {
if file, err := p.TryOpen(ctx, opener, flags); err != syserror.ErrWouldBlock {
return file, err
}
// Honor the cancellation request if open still blocks.
if canceled {
// If we were canceled but we have a handle to a host
// file, we need to close it.
if p.hostFile != nil {
p.hostFile.Close()
}
return nil, syserror.ErrInterrupted
}
cancel := ctx.SleepStart()
select {
case <-cancel:
// The cancellation request received here really says
// "cancel from now on (or ASAP)". Any environmental
// changes happened before receiving it, that might have
// caused open to not block anymore, should still be
// respected. So we cannot just return here. We have to
// give open another try below first.
canceled = true
ctx.SleepFinish(false)
case <-time.After(100 * time.Millisecond):
// If we would block, then delay retrying for a bit, since there
// is no way to know when the pipe would be ready to be
// re-opened. This is identical to sending an event notification
// to stop blocking in Task.Block, given that this routine will
// stop retrying if a cancelation is received.
ctx.SleepFinish(true)
}
}
}
// pipeOpenState holds state needed to open a blocking named pipe read only, for instance the
// file that has been opened but doesn't yet have a corresponding writer.
type pipeOpenState struct {
// hostFile is the read only named pipe which lacks a corresponding writer.
hostFile *fd.FD
}
// unwrapError is needed to match against ENXIO primarily.
func unwrapError(err error) error {
if pe, ok := err.(*os.PathError); ok {
return pe.Err
}
return err
}
// TryOpen uses a NonBlockingOpener to try to open a host pipe, respecting the fs.FileFlags.
func (p *pipeOpenState) TryOpen(ctx context.Context, opener NonBlockingOpener, flags fs.FileFlags) (*pipeOperations, error) {
switch {
// Reject invalid configurations so they don't accidentally succeed below.
case !flags.Read && !flags.Write:
return nil, syscall.EINVAL
// Handle opening RDWR or with O_NONBLOCK: will never block, so try only once.
case (flags.Read && flags.Write) || flags.NonBlocking:
f, err := opener.NonBlockingOpen(ctx, fs.PermMask{Read: flags.Read, Write: flags.Write})
if err != nil {
return nil, err
}
return newPipeOperations(ctx, opener, flags, f, nil)
// Handle opening O_WRONLY blocking: convert ENXIO to syserror.ErrWouldBlock.
// See TryOpenWriteOnly for more details.
case flags.Write:
return p.TryOpenWriteOnly(ctx, opener)
default:
// Handle opening O_RDONLY blocking: convert EOF from read to syserror.ErrWouldBlock.
// See TryOpenReadOnly for more details.
return p.TryOpenReadOnly(ctx, opener)
}
}
// TryOpenReadOnly tries to open a host pipe read only but only returns a fs.File when
// there is a coordinating writer. Call TryOpenReadOnly repeatedly on the same pipeOpenState
// until syserror.ErrWouldBlock is no longer returned.
//
// How it works:
//
// Opening a pipe read only will return no error, but each non zero Read will return EOF
// until a writer becomes available, then EWOULDBLOCK. This is the only state change
// available to us. We keep a read ahead buffer in case we read bytes instead of getting
// EWOULDBLOCK, to be read from on the first read request to this fs.File.
func (p *pipeOpenState) TryOpenReadOnly(ctx context.Context, opener NonBlockingOpener) (*pipeOperations, error) {
// Waiting for a blocking read only open involves reading from the host pipe until
// bytes or other writers are available, so instead of retrying opening the pipe,
// it's necessary to retry reading from the pipe. To do this we need to keep around
// the read only pipe we opened, until success or an irrecoverable read error (at
// which point it must be closed).
if p.hostFile == nil {
var err error
p.hostFile, err = opener.NonBlockingOpen(ctx, fs.PermMask{Read: true})
if err != nil {
return nil, err
}
}
// Try to read from the pipe to see if writers are around.
tryReadBuffer := make([]byte, 1)
n, rerr := p.hostFile.Read(tryReadBuffer)
// No bytes were read.
if n == 0 {
// EOF means that we're not ready yet.
if rerr == nil || rerr == io.EOF {
return nil, syserror.ErrWouldBlock
}
// Any error that is not EWOULDBLOCK also means we're not
// ready yet, and probably never will be ready. In this
// case we need to close the host pipe we opened.
if unwrapError(rerr) != syscall.EWOULDBLOCK {
p.hostFile.Close()
return nil, rerr
}
}
// If any bytes were read, no matter the corresponding error, we need
// to keep them around so they can be read by the application.
var readAheadBuffer []byte
if n > 0 {
readAheadBuffer = tryReadBuffer
}
// Successfully opened read only blocking pipe with either bytes available
// to read and/or a writer available.
return newPipeOperations(ctx, opener, fs.FileFlags{Read: true}, p.hostFile, readAheadBuffer)
}
// TryOpenWriteOnly tries to open a host pipe write only but only returns a fs.File when
// there is a coordinating reader. Call TryOpenWriteOnly repeatedly on the same pipeOpenState
// until syserror.ErrWouldBlock is no longer returned.
//
// How it works:
//
// Opening a pipe write only will return ENXIO until readers are available. Converts the ENXIO
// to an syserror.ErrWouldBlock, to tell callers to retry.
func (*pipeOpenState) TryOpenWriteOnly(ctx context.Context, opener NonBlockingOpener) (*pipeOperations, error) {
hostFile, err := opener.NonBlockingOpen(ctx, fs.PermMask{Write: true})
if unwrapError(err) == syscall.ENXIO {
return nil, syserror.ErrWouldBlock
}
if err != nil {
return nil, err
}
return newPipeOperations(ctx, opener, fs.FileFlags{Write: true}, hostFile, nil)
}
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